While this invention is applicable to many types of indirect heat exchange apparatus, it is particularly efficacious in an internally air cooled turbine blade utilized in a gas turbine engine. As one skilled in the gas turbine engine technology will appreciate, it is abundantly important that the pressure and amount of cooling air utilized for cooling the airfoils of the turbine rotor is maintained to a minimum. Excess use of cooling air, which is typically bled off of the compressor of the gas turbine engine, would be a deficit in terms of engine performance. Obviously, the air being bled has already had work expended thereon by the compressor and this energy, if not converted into thrust or horsepower degrades engine performance. With the high demands for good engine performance, it is easy to understand the importance of holding the amount of cooling air to a minimum.
I have found that by tailoring the performance of the heat exchanger to match the temperature characteristics of the medium in which it is in indirect heat exchange, the overall performance of the heat exchanger can be significantly enhanced. In an airfoil environment, for example, the serpentine passages internally of the airfoil are "short circuited" in order to maximize the indirect heat exchange in areas that require high heat transfer and minimize indirect heat exchange in areas where the heat transfer requirement are less. Hence, the heat exchanger that is operating in an environment that has an external temperature gradient is tailored to provide maximum heat exchange transfer potential at the critical spans and consequently, improving the efficiency of the heat exchanger results in requiring less pressure at the airfoil inlet to flow the amount of air internally of the blade to attain the required cooling of the airfoil.